The belt slot is one of what is sometimes referred to as the 'big six' in PF1.
The other members of the big six are Head, Cloak, Amulet, Ring, Armour and Weapon. That is technically seven slots, but that is because the belt slot is replaced by a headband for characters who care more about mental stats.
Any of those are reasonable candidates - for each one you would be giving up a substantial benefit; to the extent that Pathfinder generally assumes you have them.
Of those, I would suggest that head and ring are probably the least appropriate slots. Head as it is more often linked with mental stats, and Ring as the fact that you can wear two of them means it isn't as much a core part of the big six as the others (i.e. you could wear this new ring and still wear your ring of protection).
As an alternative, you could consider the Belt of Physical Might which increases both Strength and Dex? (this might not sufficiently increase your carrying capacity) - or perhaps design a more expensive belt that combines the Dex bonus with ant haul (As a GM I'd probably allow that at or near the price of the Belt of Physical Might)using these rules.